BOSTON -- One of the best compliments that you could give to David Robertson's season is that Mariano Rivera's name has not come up often, but just as much, the new closer has enjoyed watching Dellin Betances' explosion as a premier setup man.

Even Robertson, having a fine year in his own right, was wowed by Betances' appearance on Saturday at Fenway Park, when the stadium scoreboard clocked two of Betances' eighth-inning fastballs at 101 mph.

"How could you not have fun watching that?" Robertson said. "He threw 101. Wow! I'm lucky if I hit 93 and he's pumping it in at 101."

Betances' 13.22 strikeouts per nine innings in 48 relief appearances (spanning 65 1/3 innings) are the highest mark in the Majors this season, as the right-hander has learned how to harness his two-pitch repertoire into terrific results.

"I definitely think he's exceeded [expectations]," manager Joe Girardi said. "We knew he had great stuff. We knew this would be a year he had never really experienced before, in a sense, if he got on a roll. And that's what he's done."

Robertson said that Betances' electric stuff stands out, but there is still learning on the job. Betances was developed as a starting pitcher before being transitioned to the bullpen in the Minors back in 2012.

"You do have to get used to that workload," Robertson said. "It takes a little bit of time. Obviously you need to figure out your body first; that was my biggest key. When I'm throwing 70 or 65 appearances a year, some of them come on back-to-back-to-back days, you have to be ready to say when we play catch, just play a little bit.

"You just know that your arm feels good and you can go out there and do the job you're supposed to do."

Bryan Hoch is a reporter for MLB.com. Follow him on Twitter @bryanhoch and read his MLBlog, Bombers Beat. This story was not subject to the approval of Major League Baseball or its clubs.